If the A2090 has "A2090" printed on it, it's an A2090a (autoboot version). If it says "HARD DISK CONTROLLER", it's no autoboot.
The autoboot version can work quite nicely in an unaccelerated A2000, but it may be a pain to set up due to it's outdated software (predates RDB).
You can use any SCSI drive up to 512 MB, probably dirt cheap at eBay (even larger ones, but only the first 512 MB of them). Forget about MFM drives, they're slow and much more unreliable.
Just use a small OFS partition to boot from and mount the real FFS/DCFFS/SFS/whatever partition and don't set up any partition larger than 256 MB (according to
BBoAH).
